Ion implantation beam monitor

ABSTRACT

An ion implanter, the total return current between the substrate holder and flight tube is measured. Measuring the total current returned to the flight tube provides a useful indication of the total ion current in the ion beam leaving the flight tube as well as any electrons travelling back to, and being absorbed by, the flight tube. This in turn permits the quality of the ion beam post mass selection to be monitored, continuously if desired. The total current returned to the flight tube can be compared with the current measured by the beam, the latter varying rapidly with time as the beam stop is periodically occluded by the rotating substrate wheel. 
     In order to general a potential difference between the substrate holder and the flight tube, either a power supply or an active resistance such as an FET chain can be employed.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/GB99/02087 which has an Internationalfiling date of Jul. 1, 1999, which designated the United States ofAmerica.

FIELD OF THE INVENTION

This invention relates to an ion implantation beam monitor, inparticular for monitoring the quality or stability of an ion beam in anion implanter which implants ions into substrates such as semiconductorwafers.

This application claims priority under 35 U.S.C. §119 of United KingdomApplication No. 9814285.4, which has a filing date of Jul. 1, 1998, inaccordance with 37 C.F.R. §1.55(1)(ii) and which was identified inaccordance with the regulations under the Patent Cooperation Treaty as apriority document to PCT International Application No. PCT/GB99/02087from which this application is derived.

BACKGROUND OF THE INVENTION

Semiconductor devices are typically formed from a semiconductorsubstrate material into which atoms or molecules of selected dopantshave been implanted or defused. The dopant particles produce regions inthe semiconductor substrate having varying conductivity. By selectingappropriate dopant materials, the majority charge carrier may be locallyaltered within the substrate.

One preferred technique for adding dopant materials to semiconductorsubstrates uses ion implantation. This technique minimises the size ofthe device features created by the dopants within the substrate,reducing the overall size of the semiconductor device itself andincreasing operational speed.

The principles of operation of an ion implantation apparatus will befamiliar to those skilled in the art. Briefly, a plasma generatespositive ions of the selective dopant material in an ion source. Therequired positively charged ions are extracted from the ion source,which are accelerated by application of an acceleration potentialthrough a magnetic field. The magnetic field is generated by a massselection arrangement which bends the extracted ions around a curvedpath. The radius of curvature of the flight path of the ions isdependent upon the mass/charge ratio of the individual ions. The exit ofthe mass selection arrangement has a slit within it so that only ionshaving a predetermined mass/charge ratio can exit the mass selectionarrangement.

Those ions exiting the mass selection arrangement impinge upon asemiconductor substrate to be doped. Typically, this substrate will havepreviously been patterned with photo resist so that only selectedregions are doped.

The depth to which ions are implanted in the substrate will depend upontheir energy when incident upon the substrate. In order to generate ionsfor implantation, it is standard practice to provide an arrangement togenerate a desired potential difference between the “flight tube” andsample holder. The flight tube is an expression used throughout thisspecification and the claims to refer to the metalwork of the massselection arrangement which is normally all at a common potential withreference to earth. However, the term “flight tube” is also applicableto any structure where corresponding electrode arrangements arise—forexample a structure which itself has a potential referenced to earth andfrom which the potential of the ion source is in turn referenced. Forexample, the flight tube may contain an r.f. accelerator or boosterpositioned downstream of the mass selector for which the final exitelectrode of the booster is at a potential referenced to the massselection.

Usually, the target substrate is held at or near earth potential. If theflight tube is at a positive potential with respect to the sampleholder/target substrate, then the ions are accelerated post massselection. This is called “post acceleration”, and is desirable whendeep implantation of ions is required. On the other hand, if the flighttube is at a negative potential with respect to the sample holder, thenthe ions are decelerated post mass selection. This is called “postdeceleration”. Post deceleration is typically employed to produce veryshallow structures within the surface of the substrate, to decrease thesize and increase the speed of the resultant semiconductor device.

In some arrangements, it is desirable to operate the flight tube atsubstantially the same potential as the substrate holder, so that thereis no post acceleration or deceleration. This mode of operation istermed the “drift mode”.

Frequently, the cross-sectional area of the ion beam at the substrate isless than that of the substrate, in order to prevent ions in the beamfrom sputtering the walls of the apparatus and introducing impurities,and to increase doping uniformity. This necessitates scanning of eitherthe substrate relative to a fixed direction ion beam or scanning the ionbeam across a fixed substrate. In practice, it is preferable to scan thesubstrate while maintaining the ion beam in a fixed direction.

It is important when doping substrates with an ion beam from an ionimplantation apparatus that the ion current striking the substrate bemonitored. This is to ensure correct doping. If, for example, the ionbeam contains a drop-out, then the substrate will not be doped properly.Several ways of monitoring the beam have been proposed, and a Faradaydetector, such as a Faraday cup or bucket, is often employed.

In one known apparatus, the Faraday detector acts as a beam stoparranged down stream of a rotating spoked wheel. The rotating wheelcarries a plurality of substrates to be doped and is scanned across anion beam. The beam stop collects ions passing through or missing thewheel, as well as any secondary electrons generated. In anotherapparatus, a solid wheel is employed instead, and the Faraday detectoris arranged upstream of the wheel.

Each of these ion beam monitors suffer from drawbacks, however. It istherefore an object of the present invention to provide an improvedapparatus and method for monitoring the ion beam.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is providedan ion implantation apparatus comprising a holder for a substrate to beimplanted, a source of ions to be implanted in the substrate, a flighttube through which ions from the source travel towards the holder, anion accelerator arranged to supply an acceleration bias between thesource and the flight tube such that the ions are accelerated to anacceleration energy, a power supply arranged to generate a desiredpotential difference between the substrate holder and flight tube, anelectrically conductive return current path connected to conduct theentirety of the current returned to the flight tube such that the flighttube is maintained at a desired potential relative to the substrateholder, and a return current monitor arranged to provide a signalrepresentative of the return current flowing through the return currentpath.

Measuring the total current returned to the flight tube provides auseful indication of the total ion current in the beam leaving theflight tube, as well as any electrons travelling back to, and beingabsorbed by, the flight tube. This in turn permits the quality of theion beam post mass selection to be monitored, continuously if desired.Previously, for example when using a beam stop behind the substrateholder, the ion beam could only be monitored (via the beam stop current)when the ion beam was not absorbed by the substrate holder. Thisprevented drop-outs in the ion beam, for example due to arcing in theion source, from being detected if these occurred whilst the substrateswere absorbing the ion beam rather than the beam stop.

Preferably, the power supply generates a potential difference betweenthe substrate holder and flight tube such that the ions are deceleratedbetween the flight tube and the substrate holder. Although post massselection acceleration is desirable in certain cases, it is more oftenpreferable to decelerate the ions prior to impact with a substrate.

According to a second aspect of the present invention, there is providedan ion implantation apparatus comprising a holder for a substrate to beimplanted, a source of ions to be implanted in the substrate, a flighttube through which ions from the source travel towards the holder, anion accelerator arranged to supply an acceleration bias between thesource and the flight tube such that the ions are accelerated to anacceleration energy, a low resistance, electrically conductive returncurrent path connected to conduct the entirety of the current returnedto the flight tube such that the flight tube is maintained atsubstantially the same potential as the substrate holder, and a returncurrent monitor arranged to provide a signal representative of thereturn current flowing through the return current path.

Connecting the flight tube to the substrate holder via a low resistancepath means that the ions exiting the flight tube are neither acceleratednor decelerated as they approach the substrate holder.

Preferably, the ion implantation apparatus of the first and secondaspects of the present invention further comprises a beam stop arrangedto absorb a portion of the ion beam not absorbed by the substrate orsubstrate holder, and to generate a beam stop current therefrom, thebeam stop current being returned to the flight tube via the returncurrent path. In that case, the apparatus may further comprise acomparator arranged to provide a signal representative of the differencebetween the return current through the return current path and the beamstop current. Thus, the beam stop current may be compared, in real time,with the total current being returned from the substrate, substrateholder and beam stop to the flight tube.

According to a third aspect of the present invention, there is providedan ion implantation apparatus comprising a holder for a substrate to beimplanted, a source of ions to be implanted in the substrate, a flighttube through which ions from the source travel towards the holder, anion accelerator arranged to supply an acceleration bias between thesource and the flight tube such that the ions are accelerated to anacceleration energy, a beam stop arranged to absorb a portion of the ionbeam not absorbed by the substrate or substrate holder, and to generatea beam stop current therefrom, an electrically conductive controlledcurrent path connected to conduct the entirety of the current to theflight tube such that a desired potential difference is maintainedbetween the substrate holder and flight tube, and a comparator arrangedto provide a signal representative of the difference between thecontrolled current through the return current path and the beam stopcurrent.

Comparing the beam stop current with the total current returned to theflight tube allows a cross check on beam stop accuracy to be made, whenthe ion beam does not impinge upon any part of the substrate holder.

The apparatus may also include scanning means for scanning the ionsrelative to the substrate, and optionally relative to the beam stop aswell. For example, the substrate may be scanned relative to a fixed ionbeam direction.

Preferably, the substrate holder comprises a plurality of substratesupports each mounted relative to a rotatable hub. The beam stop mayinclude a Faraday-type detector.

The invention also extends to a method of implanting ions in a substrateat a desired implant energy, comprising accelerating ions to a transportenergy by supplying an acceleration bias between an ion source and aflight tube through which the ions travel, transporting the ions throughthe flight tube to the substrate, generating a return current signalrepresentative of the entirety of the current returned to the flighttube, the return current being controlled such that a desired potentialdifference is maintained between the substrate holder and flight tube,and monitoring the ions transported through the flight tube to thesubstrate based upon the signal representative of the return current.

Furthermore, there is provided a method of monitoring the quality of anion beam in an ion implantation apparatus, comprising accelerating ionsto a transport energy by supplying an acceleration bias between an ionsource and a flight tube through which the ions travel, transporting theions as in an ion beam through the flight tube to a substrate,generating a return current signal representative of the entirety of thecurrent returned to the flight tube, the return current being controlledsuch that a desired potential difference is maintained between thesubstrate holder and flight tube, and monitoring the quality of the ionbeam based upon the signal representative of the return current.

BRIEF DESCRIPTION OF THE INVENTION

The invention may be put into practice in a number of ways, one of whichwill be described by way of example only and with reference to theaccompanying drawings in which:

FIGS. 1a and 1 b show front views of a substrate holder and beam stop;

FIG. 2 shows the output of the beam stop of FIG. 1 as a function oftime;

FIG. 3 shows a schematic view of an ion implantation apparatus accordingto a first preferred embodiment of the present invention including afirst return current monitor;

FIG. 4 shows, schematically, the current paths in the ion implantationapparatus of FIG. 3;

FIG. 5 shows the output of the beam stop of FIG. 1 together with theoutput of the further beam current detector of FIG. 3; and

FIG. 6 shows an ion implantation apparatus according to a secondpreferred embodiment of the present invention including a second returncurrent monitor.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1a and 1 b show a typical substrate holder 10 looking along thelines of ions exiting the slit in the mass selection arrangement. Thesubstrate holder 10 is of the spoked wheel type and comprises aplurality of substrate supports 20, onto which substrates to be dopedmay be affixed. The substrates supports 20 are spaced equidistantly froma central hub 22 by a plurality of spokes 24. The central hub 22 isconnected to a drive 26 by a shaft 28. The drive 26, which may forexample be an electric motor, drives the shaft 28 such that the hub 22is caused to move reciprocally in the manner of an inverted pendulum.Referring to FIG. 1a, this motion is indicated by the arrows AA′. In thefollowing description, one movement of the shaft from its leftmostextension at A′, to its rightmost extension A, and back to its leftmostextension A′, is termed a substrate scan.

In addition to its reciprocal motion, the hub is also rotated about itsaxis, as indicated by the second arrow B in FIG. 1a. Thus, the ion beam,which normally follows a fixed, linear trajectory once it exits the massselection apparatus, is caused to scan across the plurality ofsubstrates held on the substrate supports 20 by the reciprocating androtating movement of the substrate holder 10.

In this preferred embodiment, the ion beam is aimed at a beam stop 30arranged downstream of the substrate holder 10, as shown in FIGS. 1a and1 b. The beam stop 30 includes a Faraday type current detector 40,connected to ancillary circuitry 50 which generates a beam stop currentrepresentative of the charge built up in the Faraday type currentdetector 40.

When the shaft 28 of the substrate holder is at its furthest point oftravel away from the beam stop 30, as shown in FIG. 1a, the ion beam,which is directed at the Faraday type current detector 40, strikes onlythe current detector itself, and completely misses the rotatingsubstrate support 20. The beam stop current is at this point similar tothe actual ion beam current. However, as the shaft 28 of the substrateholder 20 moves back towards the beam stop 30, the current detector 40becomes obscured by the plurality of rotating substrate supports as theypass in front of it. The apparent current measured by the beam stop thusreduces as the ion beam is absorbed instead by the substrates upon thesubstrate supports 20.

It will be understood that other substrate holder arrangements arecontemplated. For example, a solid wheel (rather than a spoked wheel)may be used. Furthermore, the substrate holder may move reciprocally upand down (or side to side) in a straight line rather than in an arc asdescribed above.

It will be understood that other substrate holder arrangements could beused.

FIG. 2 shows a schematic plot of beam stop current I_(beam) againsttime. Over the period marked P in FIG. 2, the beam-current is a maximum,and occurs when the shaft 28 of the substrate holder 10 is at itsfurthest point A′ away from the beam stop 30. As the shaft 28 moves backtowards the beam stop 30, the substrate supports 20 partially block thepassage of the ion beam to the current detector 40. At this point, theion beam is able to pass wholly through the gaps between the substrates,however, and the beam current thus becomes a series of negative pulsesbetween I_(beam)=I_(max) and I_(beam)<I_(max), as shown in region Q ofFIG. 2.

The shaft 28 continues to move towards the beam stop 30, and the minimumcurrent in the current pulses reduces to zero as the ion beam is whollyabsorbed by the substrates as they pass. Furthermore, the maximumcurrent in each pulse reduces, as the height of the ion beam is greaterthan the spacing between substrates and a part of the ion beam iscaptured by the substrates at all times. This is shown in region R ofFIG. 2.

As the substrate supports 20 pass to the left of the current detector 40(when viewed along the line of the ion beam in the arrangement of FIGS.1a and 1 b) the measured current increases again. A subsidiary maximumis reached when the ion beam is partially obscured only by the spokes 24of the substrate holder 10. This is shown at S in FIG. 2.

The shaft 28 then starts to return back towards its point of furthesttravel (A′) from the beam stop 30 and the current eventually reaches aprimary maximum again, at T, as shown in FIG. 2. It will be appreciatedthat, in fact, the widths of the pulses (several milliseconds each) areseveral orders of magnitude less than the time for one scan cycle, andin practice only the carrier wave (indicated by a broken line in FIG. 2)is detected, rather than the modulated output of FIG. 2. This is becausethe current to voltage converter, explained below, includes a low passfilter in series with its output, that rolls off at 80 Hz. This filtersout the pulses shown in FIG. 2, which typically have a frequency between350 Hz and 550 Hz.

The use of a beam stop in the arrangement of FIG. 1 thus preventsaccurate measurement of ion beam current, and hence meaningful ion beamstability monitoring, over most of the substrate doping operation.

Referring now to FIG. 3, an ion implantation apparatus 100 is shown. Theapparatus comprises an ion beam source 110, such as a Freeman or Bernassource, which is supplied with ions of a substance which is to beimplanted into a substrate. The ions are extracted from the ion source110 by an extraction electrode assembly 120 which is in electricalcontact with a flight tube 130. The flight tube 130 is electricallyisolated from the ion source 110 and a high tension power supply 140applies a potential difference between the flight tube 130 and ionsource 110.

This potential difference causes positively charged ions to be ejectedfrom the ion source 110 into the flight tube 130. The flight tube 130includes a mass analysis arrangement comprising a mass analysing magnet150 and a mass selection electrode 160. Upon entering the mass analysisapparatus within the flight tube 130, the electrically charged ions aredeflected by the magnetic field of the mass analysis magnet 150. Theradius of curvature of each ion's flight path is defined, for a constantmagnetic field, by the mass/charge ratio of the individual ions.

A mass selection slit 170 is provided within the mass selectionelectrode 160, so that only ions having a chosen mass/charge ratio canexit the mass analysing apparatus. Those ions passing through the slit170 enter a tube 180 which is electrically connected to, and integralwith, the flight tube 130.

The mass selected ions exit the tube 180 and strike a semiconductorsubstrate 190, mounted upon a substrate holder 10. The substrate holder10 holds a plurality of substrates, as has been described in relation toFIGS. 1a and 1 b. Located behind (i.e, downstream of) the substrateholder 10 is a beam stop 30.

To maintain the beam current at an acceptable level, a minimum ionextraction energy of about 10 keV is employed. The ions are maintainedat this energy throughout the flight tube until they emerge from thetube 180 downstream of the slit 170. It is often desirable for theenergy at which the ions impact the substrate 190 to be considerablylower than the 10 keV extraction energy. For example, the energy atimpact on the substrate may desirably be as low as several hundredelectron volts. Thus, a reverse bias voltage must be applied between thesubstrate 190 and the flight tube 130. The flight tube is, of course, ata negative potential relative to the ion source 110 by virtue of theregulated high tension power supply 140.

The substrate holder 10 and beam stop 30 are contained within a housing200 which is mounted relative to the flight tube 130 by insulatingstandoffs 210. Both the beam stop 30 and substrate holder 10 areconnected to the flight tube 130 via a deceleration power supply.Normally, the beam stop and substrate holder are held at a common groundpotential so that, in order to decelerate the positively charged ions,the deceleration power supply 220 generates a negative potential withrespect to the grounded substrate holder beam stop at the flight tube130.

In order for the deceleration power supply 220 to maintain a regulatedvoltage between the substrate holder/beam stop and flight tube, it isimportant to ensure that a forward current flows through thedeceleration power supply 220. This is achieved by connecting adeceleration supply load resistance 240 in parallel with the powersupply 220.

In order to provide cooling to assemblies in the beamline and ion sourceareas of the implanter, a closed circuit cooling water flow is requiredfrom a heat exchanger located at ground potential. The flow and returnpipes must cross the post mass acceleration or deceleration voltagegaps. The wafer is slightly electrically conductive and part of thereturn current flow from the substrate passes through these pipes. Thisis a further effective load resistance in parallel with the decelerationsupply 220. Although the current returned through the cooling pipes istypically negligible, the current through the water used to cool thesubstrate holder (which is usually deionized) will not necessarily benegligible. For example, when high post mass acceleration ordeceleration voltages are employed, a cooling water current of severalmA may arise. To take this into account FIG. 3 shows a cooling systemresistance 250, in parallel with the deceleration supply load resistance240 and the deceleration power supply 220.

The current flowing through the deceleration supply load resistance 240will then be the sum of the forward current through the decelerationpower supply 220 (I_(decel)) and the net beam current I_(beam) absorbedby both the substrate 190 and beam stop 130 minus a small cooling systemwater current.

The output of the beam stop 30 is monitored by a first current monitor340, which generates a voltage signal representative of the beam stopcurrent. This voltage signal is connected to one input of a comparator350, as will be described in more detail below.

The ion implantation apparatus 100 also contains a second currentmonitor 260 arranged in the path of the total current (the sum of thebeam and deceleration currents) as it returns to the flight tube 130.This second current monitor 260 also generates a voltage signalV_(TOTAL), which indicates the total current returning to the flighttube. In one embodiment, the signal V_(TOTAL) may be measured directlywithout comparing it to the beam stop current.

Alternatively, the signal V_(TOTAL) is fed to a second input of thecomparator 350. Thus, the comparator 350 generates an output V_(DIFF)representative of the difference between the beam stop currentI_(beam stop) and the total current I_(TOTAL) returned to the flighttube 130.

Referring now to FIG. 4, a schematic diagram of the current flow and thetwo current monitors is shown. Features common to FIGS. 3 and 4 arelabelled with similar numerals.

The incident ion beam 300 impinges upon the substrate holder 10 and beamstop 30. It will be appreciated that, whilst in FIG. 4 the beam is shownscanned relative to a stationary wheel and beam stop, it is in practicepreferable to scan the wheel whilst maintaining the ion beam 300stationary and directed towards the beam stop.

The output of the beam stop 30 is connected to ground potential via afirst current monitor shown generally at 340. The first current monitor340 includes a current-to-voltage converter employing a firstoperational amplifier 360. The output of the beam stop 30 is fed to theinverting input of the operational amplifier 360, the non-invertinginput thereof being grounded. In parallel with the first operationalamplifier 360 is a first feedback resistor 370. As the non-invertinginput of the first operational amplifier 360 is at ground potential, theinverting input is at a virtual earth potential. The inputs of anoperational amplifier do not draw a current, and the first currentmonitor 340 therefore acts as a current-to-voltage converter. Thecurrent flows through the operational amplifier from ground via powerrails 362,364.

The output of the beam stop current to voltage converter is filtered bya low pass filter (not shown) to remove the relatively high frequencypulses caused by the preferred “wheel” shape of the substrate holder asit cuts the ion beam. Active or passive devices can be used to filterthe signal and suitable ways of doing this will be apparent to theskilled person.

The voltage output of the first current monitor is connected to adifferential amplifier 350, as previously described in connection withFIG. 3.

The total current from the substrate holder 10 and beam stop 30 passesthrough the parallel arrangement of the deceleration power supply 220,the deceleration supply load resistance 240, and any cooling systemresistance 250. The total current I_(TOTAL) is fed to a currenttermination plate 310 which in turn is attached to a second currentmonitor 260 which operates in a similar manner to the first currentmonitor 340. In particular, the second current monitor includes a secondoperational amplifier 320, having in inverting input connected to thecurrent termination plate 310. The second current monitor 260 also has asecond feedback resistor 330 arranged in parallel with the secondoperational amplifier 320. Again, current flow through the secondoperational amplifier 320 is maintained through a DC supply (not shown)attached to the power rails of the second operational amplifier 332,334.

The advantages of monitoring the total current returning to the flighttube, instead of, or as well as, the current from the beam stop 30, maybe seen from FIG. 5a, which shows the measured beam current as afunction of time. The curve labelled I_(beam stop) is the currentmeasured only by the first current monitor 340 (indicative of thecurrent from the beam stop). The curve marked I_(TOTAL) is the totalcurrent returned from the beam stop 30 and substrate holder 10 to theflight tube. In other words, the curve marked I_(TOTAL) should bebroadly indicative of the ion beam current at the point when it impactsthe substrate holder/beam stop assembly. Any arcing, for example, in theion source will manifest itself as a drop-out in the ion beam. This inturn may be monitored by monitoring I_(TOTAL). At any time during theimplantation cycle, a qualitative indication of ion beam integrity maythen be obtained. In particular, the voltage signal which is an outputof the current monitor 260 allows wide band stability monitoring of theion beam.

Furthermore, the problem of ripples in the current measured by the beamstop is largely avoided with the apparatus of the present invention.I_(TOTAL) is slightly distorted due to back streaming electronsgenerated when the ion beam is striking the substrates. For positivelycharged ions, any electrons liberated from the substrates areaccelerated away from the substrates (for ion deceleration), thus addingto the current returned to the flight tube 130. The beam stopeffectively traps secondary electrons, however, and there are no backstreaming electrons to augment the current when the substrate holderdoes not occlude the beam.

FIG. 5b shows the output of the differential amplifier 350. When the ionbeam is entirely incident upon the beam stop 30, the beam stop currentsubstantially equals the current being returned to the flight tube, i.e.I_(BEAM STOP)≅I_(TOTAL). Thus, the differential output of the amplifier350 is approximately zero.

As the beam begins to impinge upon the substrates 190, the beam stopcurrent reduces, as explained in relation to FIG. 2. The total currentreturned to the flight tube does, however, not reduce by the sameamount, and the output of the differential amplifier 350 rises.

When the beam moves off the substrates 190, and the beam stop is onlypartially obscured by the substrate holder spokes 24, the beam stopcurrent rises again and the output of the differential amplifier 350reaches a subsidiary minimum. The output of the differential amplifier350 reaches another maximum as the beam passes back over the substrates190 and the beam stop current drops towards zero once more. Finally, asthe ion beam coincides with the beam stop once more, the output of thedifferential amplifier drops to approximately zero.

An alternative embodiment of the present invention is shown withreference to FIG. 6. Once again, parts of the apparatus common to FIGS.3, 4 and 6 are labelled with similar reference numerals.

As shown in FIG. 6, rather than employing a deceleration power-supply, avariable resistance 400 is placed in the current path which returns theion beam current from the substrate holder 10 and beam stop 30 back tothe flight tube 130. Although the variable resistance 400 may consist ofpassive devices, it is preferable to use a series of active devices,such as field effect transistors (FET's). The manner of operation of thedevice of FIG. 6 is described in more detail in GB 9523982.8. Briefly,the potential difference between the substrate holder/beam stop(normally held at ground potential) and the flight tube 130 iscontrolled by varying the resistance of a chain of FET's connected inseries between the substrate holder/beam stop (at ground potential) andthe flight tube. This is done by measuring the voltage across the FETchain with a potential divider, buffering it and comparing it with areference voltage V_(ref) in a differential amplifier. The error signal(i.e, the amplified difference between the desired decelerationpotential and the actual deceleration potential as measured by thepotential divider) adjusts the effective resistance of the FET chain.

The potential drop across the FET chain, V_(TOTAL), is indicative of thetotal current returned to the flight tube 130. In one embodiment, thisis fed to a comparator, such as a differential amplifier 350 (see FIG.6). The other input to the differential amplifier 350 is a voltagerepresentative of the beam stop current. This is derived from a beamstop current monitor 340. The output of the differential amplifier 350is similar to that shown in FIG. 5b.

As with the apparatus shown in FIG. 3, the voltage signal V_(TOTAL) maybe measured directly, rather than being compared with the beam stopcurrent signal.

Although a differential amplifier has been described in connection withFIGS. 3 and 6, it will be understood that a software comparison may bemore suitable in some circumstances. In the case of post mass selectionacceleration or deceleration (as opposed to the “drift mode”), V_(TOTAL)and V_(BEAMSTOP) may be separated by kilovolts and here, a softwarecomparison would be preferred.

What is claimed is:
 1. An ion implantation apparatus comprising a holderfor a substrate to be implanted, a source of ions to be implanted in thesubstrate, a flight tube through which ions from the source traveltowards the holder, an ion accelerator arranged to supply anacceleration bias between the source and the flight tube such that theions are accelerated to an acceleration energy, a power supply arrangedto generate a desired potential difference between the substrate holderand flight tube, an electrically conductive return current pathconnected to conduct the entirety of a return current to the flight tubewhich is required to maintain said desired potential difference, and areturn current monitor arranged to provide a signal representative ofthe return current flowing through the return current path.
 2. An ionimplantation apparatus as claimed in claim 1, in which the power supplygenerates a potential difference between the substrate holder and flighttube such that the ions are decelerated between the flight tube and thesubstrate holder.
 3. An ion implantation apparatus comprising a holderfor a substrate to be implanted, a source of ions to be implanted in thesubstrate, a flight tube through which ions from the source traveltowards the holder, an ion accelerator arranged to supply anacceleration bias between the source and the flight tube such that theions are accelerated to an acceleration energy, a low resistance,electrically conductive return current path connected to conduct theentirety of a return current to the flight tube which is required tomaintain the flight tube at substantially the same potential as thesubstrate holder, and a return current monitor arranged to provide asignal representative of the return current flowing through the returncurrent path.
 4. An ion implantation apparatus as claimed in claim 1further comprising a beam stop arranged to absorb a portion of the ionbeam not absorbed by the substrate or substrate holder, and to generatea beam stop current therefrom, the beam stop current being returned tothe flight tube via the return current path.
 5. An ion implantationapparatus as claimed in claim 4, further comprising a comparatorarranged to provide a signal representative of the difference betweenthe return current through the return current path and the beam stopcurrent.
 6. An apparatus claimed in claim 1, further comprising scanningmeans for scanning the ions relative to the substrate.
 7. An apparatusas claimed in claim 6, in which the scanning means is arranged to scanboth the substrate and the beam stop with the ions.
 8. An apparatusclaimed in claim 6, in which the scanning means scans the substraterelative to a fixed ion beam direction.
 9. An apparatus as claimed inclaim 1, in which the substrate holder comprises a plurality ofsubstrate supports each mounted relative to a rotatable hub.
 10. Anapparatus as claimed in claim 1, in which the beam stop includes aFaraday-type detector.
 11. An apparatus as claimed in claim 1, in whichthe flight tube includes mass selection means arranged to select only aproportion of the accelerated ions to be implanted into the substratebased upon the mass and charge of the said accelerated ions.
 12. An ionimplantation apparatus comprising a holder for a substrate to beimplanted, a source of ions to be implanted in the substrate, a flighttube through which ions from the source travel towards the holder, anion accelerator arranged to supply an acceleration bias between thesource and the flight tube such that the ions are accelerated to anacceleration energy, a beam stop arranged to absorb a portion of the ionbeam not absorbed by the substrate or substrate holder, and to generatea beam stop current therefrom, an electrically conductive controlledreturn current path connected to conduct the entirety of a returncurrent to the flight tube which is required to maintain a desiredpotential difference between the substrate holder and the flight tube,and a comparator arranged to provide a signal representative of thedifference between the controlled current through the return currentpath and the beam stop current.
 13. An apparatus claimed in claim 12,further comprising scanning means for scanning the ions relative to thesubstrate.
 14. An apparatus as claimed in claim 13, in which thescanning means is arranged to scan both the substrate and the beam stopwith the ions.
 15. An apparatus claimed in claim 13, in which thescanning means scans the substrate relative to a fixed ion beamdirection.
 16. An apparatus as claimed in claim 12, in which thesubstrate holder comprises a plurality of substrate supports eachmounted relative to a rotatable hub.
 17. An apparatus as claimed inclaim 12, in which the beam stop includes a Faraday-type detector. 18.An apparatus as claimed in claim 12, in which the flight tube includesmass selection means arranged to select only a proportion of theaccelerated ions to be implanted into the substrate based upon the massand charge of the said accelerated ions.
 19. A method of implanting ionsin a substrate at a desired implant energy, comprising accelerating ionsto a transport energy by supplying an acceleration bias between an ionsource and a flight tube through which the ions travel, transporting theions through the flight tube to the substrate, generating a returncurrent signal representative of the entirety of the current returned tothe flight tube, the return current being controlled such that a desiredpotential difference is maintained between the substrate holder andflight tube, and monitoring the ions transported through the flight tubeto the substrate based upon the signal representative of the returncurrent.
 20. A method as claimed in claim 19, further comprisingabsorbing in a beam stop a portion of the ion beam not absorbed by thesubstrate or substrate holder, and generating a beam stop current signalrepresentative of that portion of the ion beam not absorbed by thesubstrate.
 21. A method as claimed in claim 20, further comprisingcomparing the beam stop current signal with the return current signal.22. A method as claimed in claim 19, further comprising scanning the ionbeam relative to the substrate.
 23. A method of monitoring the qualityof an ion beam in an ion implantation apparatus, comprising acceleratingions to a transport energy by supplying an acceleration bias between anion source and a flight tube through which the ions travel, transportingthe ions as in an ion beam through the flight tube to a substrate,generating a return current signal representative of the entirety of thecurrent returned to the flight tube, the return current being controlledsuch that a desired potential difference is maintained between thesubstrate holder and flight tube, and monitoring the quality of the ionbeam based upon the signal representative of the return current.
 24. Amethod as claimed in claim 23, further comprising selecting only aproportion of the accelerated ions to be transported to the substratebased upon the mass and charge of the said accelerated ions.
 25. Amethod as claimed in claim 23, in which the quality of the ion beam ismonitored only after the proportion of the accelerated ions to betransported has been selected.